"Sustainable agriculture integrates three main goals--environmental health, economic profitability, and social and economic equity... Sustainability rests on the principle that we must meet the needs of the present without compromising the ability of future generations to meet their own needs."....http://www.sarep.ucdavis.edu/concept.htm
What this page is saying is that sustainability is looking at the whole system - people, animals, water, air, land, plants, our environmental system, our economic system, and our social system - as a whole, to devise a system that will work now and far into the future. Otherwise the human race will probably die off within the next few centuries, possibly taking much of the other life-forms on earth with it (global warming, pollution, over-crowding, etc).
So yes, I would say that is a very different concept than organic. (Eat organic!! Yay!)